1. Field of the Invention
The present invention relates to a novel cladding material for plastic optical fibers and plastic optical fibers comprising the material.
More particularly, the present invention relates to a cladding material for plastic optical fibers which has a low refractive index and is excellent in transparency, and is characterized with good mechanical properties, compatibility with core material, adhesion to core material, etc.; and relates to plastic optical fibers comprising said cladding material which have a low loss and a large numerical aperture and which are excellent in heat resistance, mechanical properties, processability, flexibility, etc.
2. Discussion of Related Art
In recent years, optical communication techniques using optical fibers as information channels have advanced rapidly and has already been made practicable.
Optical fibers are divided into two groups, i.e., silica glass optical fibers and plastic optical fibers. Compared with silica glass optical fibers, plastic optical fibers are more flexible and have a larger diameter and a larger numerical aperture, and termination and joining between the fibers are easier. Therefore, the application of plastic optical fibers in the fields of wiring in instruments, short-distance communications, sensors and light guides, have been started.
As resins with excellent transparency and processability for plastic optical fibers, there have heretofore been known, for example, polystyrene resins, polycarbonate resin, and polymethyl methacrylate resin.
Polystyrene resins have a refractive index of about 1.59 and good transparency, and hence plastic optical fibers thereof using polymethylmethacrylate resins as cladding materials have been put to practical use. The polystyrene resins, however, are disadvantageous in that they are poor characterized with flexibility and weather resistance properties, and therefore they are not always the most suitable as materials for plastic optical fibers.
Polycarbonate resins have good transparency and a high glass transition temperature of about 150.degree. C., and hence their employment as core materials of optical fibers having a heat resistance of about 120.degree. C. has been considered.
On the other hand, polymethyl methacrylate resins are excellent in transparency but have a relatively low refractive index of about 1.49. Therefore, when a polymethyl methacrylate resin is used as the core material, a resin having a still lower refractive index of about 1.40 should be selected as the cladding material.
As resins having a low refractive index, fluorine-containing methacrylate resins have been known (Japanese Patent Publication No. 43-8978). However, although these resins are advantageous in that they are amorphous and have good transparency and a low refractive index of about 1.41, they have the following defects. Since fluorine-containing methacrylate resins are insufficient in their adhesion to polymethyl methacrylate resins, bending easily disturbs the core-cladding interface of the plastic optical fiber which uses this resin as the cladding material, resulting in a large light loss. Since fluorine-containing methacrylates are hard and brittle, the plastic optical fiber which uses this resin as a cladding material is poor in mechanical strength and flexibility. Because of the insufficient slippage of bare fiber on one another, the workability at the time of bundling and processing for fabricating a light guide is low.
There has also been disclosed a plastic optical fiber using, as the cladding material, a vinylidene fluoride-tetrafluoroethylene copolymer containing 60 to 80 mole % of vinylidene fluoride units (Jap. Pat. Appln. Kokai (Laid-Open) No. 50-20737). This copolymer is advantageous in that it has a low refractive index of about 1.41 and is excellent in its adhesion to polymethyl methacrylate resins and has good mechanical properties. However, since both vinylidene fluoride and tetrafluorethylene form crystalline polymers, control of the crystallizability by copolymerization is limited and crystallizability remains in the copolymer. Therefore, when light is totally reflected on the core-cladding boundary surface, a portion of the light is scattered and only plastic optical fibers having a relatively high transmission loss can be obtained. Moreover, the copolymer has a defect in that under a temperature condition of 80.degree. C. or higher, the crystals would grow which would lower the transparency, and result in a large increase of attenuation.
There have also been disclosed optical fibers which use vinylidene-fluoride-tetrafluoroethylenehexafluoropropylene copolymers containing less than 45 mole % of tetrafluoroethylene units and 1 to 30 mole % of hexafluoropropylene units [Japanese Pat. Appln. Kokai (Laid-Open) No. 54-80758]. The hexafluoropropylene units serves mainly to control the crystallizability. This copolymer is excellent in its adhesion to polymethyl methacrylate resins and has a relatively high transparency. Plastic optical fibers which use this copolymer as the cladding material and polymethyl methacrylate resin as the core material, initially have a low attenuation. However, when these plastic optical fibers are coated with polyethylene to form a cord and placed in a thermohygrostat having a temperature of 85.degree. C. and a humidity of 95%, the values of the attenuation would be further increased.
There have also been disclosed optical fibers which uses vitreous silica or a polymethyl methacrylate resin as the core material and a vinylidene fluoridehexafluoroacetone copolymer as the cladding material (Jap. Pat. Appln. Kokai (Laid-Open) No. 61-22305, U.S. Pat. No. 4687295Jap. Pat. Appln. Kokai (Laid-Open) Nos. 61-190304 and 62-96908). However, although a vinylidene fluoride-hexafluoroacetone copolymer has a high compatibility with polymethyl methacrylate resins, it is disadvantageous in that it retains a considerable crystallizability and hence is poor in transparency, resulting in a significant attenuation. There have also been disclosed copolymers comprising hexafluoropropylene or trifluoroethylene as a third comonomer for controlling the crystallizability (Jap. Pat. Appln. Kokai (Laid-Open) Nos. 63-21606, 63-66509 and 63-143509). However, when the components, composition or melt index of a ternary copolymer is different from or outside the respective ranges of the components, composition and melt index range of the present invention, employment of the ternary copolymer can yield only plastic optical fibers having such a low heat resistance that the loss continues to increase at high temperatures.
Thus, conventional cladding materials for plastic optical fibers and plastic optical fibers comprising such materials, have both merits and demerits, and there has not yet been realized a plastic optical fiber which is excellent in all respects, for example, attenuation, flexibility, mechanical properties and heat resistance.